1. Field of the Invention
The present invention generally relates to an automatic transmission control system, and more particularly to an automatic transmission control system used in cooperation with an acceleration slip control system for preventing slippage of the driving wheels that tends to happen during starting and acceleration.
2. Description of Related Art
An acceleration slip control system is known which prevents slippage of the driving wheels of a vehicle during starting and acceleration, so that the straight line stability and accelerating ability of the vehicle can be improved. Generally, the above-mentioned acceleration slip control system employs feedback control of the driving torque. In such a feedback control system, a target value of the driving wheel speed is defined, and the driving torque is controlled so that the quantity of slip defined as the difference between the actual driving wheel speed and the target value is equal to or less than a threshold value. Hereinafter, the above-mentioned feedback control of the driving torque is referred to as a traction control (TRC). A means for controlling the driving torque is made up of a main throttle valve cooperating with manipulation of the accelerator pedal, and a sub-throttle valve driven by a driving means, such as a stepping motor. When the vehicle slips, the acceleration slip control system drives the sub-throttle valve towards the closed state. In this manner, the driving torque is reduced and wheel slippage can be prevented.
The automatic transmission control system used in cooperation with the above acceleration slip control system performs, when the TRC is OFF, a shift control suitable for the current driving conditions which can be detected from the opening ratio of the main throttle valve and the vehicle speed. Information indicating the opening ratio of the main throttle valve is obtained from the sensor output signal of a throttle valve opening ratio sensor, and information indicating the vehicle speed is obtained from the sensor output signal of a speed sensor. When the TRC is ON, the shift control is carried out on the basis of an estimated throttle valve opening ratio and the vehicle speed. The estimated throttle valve opening ratio can be obtained from an equivalent value corresponding to the quantity of cylinder intake air. More particularly, the above equivalent value can be expressed as Q/N, where Q denotes the intake air quantity, and N denotes the engine speed. The above-mentioned automatic transmission control system is disclosed in, for example, Japanese Laid-Open Patent Publication No. 1-218932.
It can be said that the shift control based on the estimated throttle valve opening ratio and the vehicle speed is substantially equivalent to a shift control based on the sub-throttle valve opening ratio.
There is a disadvantage in the shift control based on the main valve opening ratio. When the accelerator pedal is deeply depressed in a state in which the vehicle is on a slippery road like an iced road, the engine speed at the time of up-shifting is very high even when the TRC is ON. Up-shifting causes a large amount of wheel slip and hence the vehicle condition becomes unstable.
There is a disadvantage in the shift control based on the sub-throttle valve opening ratio. Depression of the accelerator pedal does not rapidly cause down-shifting. If the driver continues to depress the accelerator pedal, a down-shifting change will abruptly take place (at the end of the shifting process based on the sub-throttle valve opening ratio). In other words, the driver cannot predict when down-shifting will take place and hence feels uncomfortable.